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1. Preparation and Application

This article provides information about the application of binder systems for NOCOLOK Flux.

Solvay offers three concepts for flux binder application:

NOCOLOK Binder (water-soluble) / NOCOLOK Thickener (water-soluble)

NOCOLOK System Binder (water-based)

NOCOLOK Flux plus Binder Mixture (water-based)

These products can be used in water-based NOCOLOK Flux slurries to improve flux particle adhesion. This is of particular interest for fluxing of pre-formed components prior to assembly in order to reduce flux fall-off and dust formation. Binders are also helpful to pre-coat certain areas with specific flux loads. All binder mixtures can be applied on external and internal surfaces.

During the brazing cycle, these binders will completely evaporate (mostly between 350 and 400°C). When used as described below, there will neither be detrimental interactions between the binder and the flux, nor between the binder and the aluminum surfaces. Trials have shown that even at four-times the standard flux load with a binder mixture there was still no surface discoloration after brazing.

2. General Comments

The surface areas to be coated with binder mixtures must be free of lubricants, oils, dirt, and dust. Means of application include spraying, dipping, and brushing.

When binders are used for flux application, the recommended flux load for good brazing results is the same as it is for the standard process (i.e. between 3 – 5 g/m2). The thickness of the binder coating is usually between 10 – 30 μm.

Drying can be done in air – requiring approximately 15 – 20 minutes at room temperature for the coating surface – and 50 – 60 minutes before the parts can be handled. Oven and forced convection drying is feasible too: at 50 – 80 °C, parts will dry within 5 – 20 minutes.

Please refer to the MSDS for detailed information regarding the safe handling of the product.

3. Preparation of Binder Mixtures

For all binder concepts and preparations, the mixtures should be prepared or opened immediately before consumption.

To prepare a mixture free of agglomerates and to achieve best coating results, the following procedures must be observed for either binder concepts:

NOCOLOK Binder / NOCOLOK Thickener

45 parts (wt%) de-ionized water (as used for preparing standard flux slurries) is mixed thoroughly with

If necessary, the mixtures can be passed through a sieve prior to use. This will remove any potential agglomerates.

Prior to use the flux powder in the mixture must be suspended. The thickener will prevent the flux powder from settling too fast, however, when stored for some time or diluted, the mixture must be well shaken before spraying.

The binder component is activated by oxygen from air. Once sprayed and dried, the product cannot be recycled or reused.

Any remaining flux / binder mixture should be stored in an airtight and sealed container. We recommend consuming the mixtures within one week after mixing.

4. Additional Information:

NOCOLOK Binder, -Thickener, and –System Binder are compatible for standard NOCOLOK Flux, -LM Flux, -Cs Flux, and -Sil Flux. They are not suitable for NOCOLOK CB Flux and -Zn Flux due to chemical reactions between these fluxes and the ingredients.

The formulations (mixing ratios) provided in Solvay’s technical information sheets and brochures are intended as general recommendations – They provide the best basis for automated spray application and have been tested with good brazing results.

The recipes can be adjusted to specific application needs by changing the mixing ratios within certain ranges.

A well balanced ratio of binder and thickener to flux in the mixtures is important for good brazing performance:

Higher binder ratios result in a harder coating layer and stronger flux adhesion. But they require more care for the binder removal step.

Very high binder and/ or thickener ratios increase the organic content in the mixture – which may result in carbon residues (discoloration) after brazing.

It is possible to reduce and/ or to increase the water content of the mixtures – resulting in higher respectively lower viscosity.

Water dilution will cause less wetting action and reduced adhesion.

A surfactant (wetting agent) is part of the binder formulation – providing uniform coating, and – compensating (to some extent) for surfaces not cleaned prior to application.

Thickener is used for adjusting the viscosity and to keep the flux powder longer in suspension – This provides better performance in spray application. Nevertheless, formulations can be prepared and used without the addition of thickener.

Cleaning before binder-based flux application is recommended – but not mandatory.

A clean surface can be coated more easily and the flux adhesion will be better.

Higher surfactant levels can compensate for some contamination – but result in more foaming.

5. Binder Flux Mixing Ratios

The standard composition is 35% NOCOLOK Flux, 15% NOCOLOK Binder, 5% NOCOLOK Thickener and 45% water. If a product with lower flux ratio is wanted (i.e., with only 10% flux), the composition must be modified. Right now, we are proposing 10% flux, 8% binder, 2% thickener and 80% water. There is only limited experience with this composition, and we are a little concerned. The reasons for our concerns are as follows:

With 35% flux, the ratio of flux to binder/ thickener on the surface of the headers is sufficient to combat the effects of the high organic content. Also, 15% binder has reasonable adhesion characteristics.

At 10% flux, the ratio of flux to binder/ thickener must be modified; otherwise there may not be sufficient flux to combat the high organic content. This is why we propose to reduce the binder and thickener to 8% and 2%, respectively. In other words, too much binder/ thickener and not enough flux may lead to black deposits on the headers after brazing and/ or difficulties in brazing.

6. Warehousing Considerations and Shelf Life

Under standard storage conditions, the shelf life is up to 12 months and probably longer. Standard storage conditions means that the product was stored at less than 30°C, as suggested in the MSDS.

The binder product can be stored at a temperature higher than 30°C, but the shelf life will shorten due to premature aging. Therefore, we recommend that the binder products be consumed within six months, if the storage temperature is a constant 40°C. This is not based on experimental data, but on general knowledge of water based polymer systems and adhesives. Any product stored at a temperature higher than 40°C should be consumed more quickly.

Under no circumstances should the binder products, in their original packaging, be exposed to a temperature of 60°C or above. We suspect that polymerization will occur, agglomerates will form and the performance will drop.

7. Thermo-Gravimetric Analysis (TGA) for Binder Flux

8. Recommendations for Reducing Costs

Is not possible to only mix the binder, thickener and flux and just add the water on site. Without the water, the flux/ thickener/ binder mixture forms a rubbery-like substance that is very difficult to work with.

The least expensive option is to purchase the binder and thickener separately and do mixing of all ingredients on site. The most convenient option is to have a ready-mix, ready-to-use product supplied.

Over the last 15 years, selective prefluxing – also called paint fluxing or binderbased fluxing – has evolved as an alternative method for applying flux powder in the aluminium brazing industry. There are many activities to define process parameters of fluxing with adhesives.

The first part of this paper outlines key features of prefluxing. The methodology for measurements of physical characteristics of binders and paint flux mixtures are described. General rules for behaviour of flux paints in brazing process are discussed together with some examples of flux paint features.

In the second part a case study is shown to illustrate common challenges when brazing with flux paint. The third and last part of this paper provides a cost comparison as guidance for choosing the right fluxing method for two different cases, one being extremely negative and the second as a positive case.

4. Flux Paint Risk Point

The fundamental condition for successful brazing is having a substrate surface which is well wetted by a molten filler alloy. Any binder as an organic compound contains carbon. This carbon must be removed before brazing, otherwise it will form a very thin deposit on the substrate surface preventing wetting by the molten filler alloy2. When the flux paint is deposited on open surface areas, like for example on the radiator headers, there is no issue to remove the binder by evaporation at higher temperature. However, when the flux paint is applied in enclosed spaces, like for example on inner surfaces of condenser manifolds, the removal of binder becomes challenging. Proper de-bindering conditions must allow the binder fumes to escape completely. Residual binder – like any other trace organic – carbonizes during the brazing cycle – causing surface discoloration and potentially poor brazing.

Fig. 5 shows a leak in the tube apex. The condenser manifold was made of two halves: cover and header. Only the cover part was prefluxed by immersion. Composition of the flux paint: flux ~ 30%; carrier (acetone) ~ 62%; binder (polymethyl methacrylate) ~ 12%.

The gap size between the tube and manifold is in the range of 40µm, which is well within the maximum recommended tolerance.

Fig. 6: Dark field image of the non brazed tube to manifold joint.

Dark field image of the non brazed joint showed that the gap is filled with transparent substance. Investigation by Scanning Electron Microscope revealed that the joint was completely filled with post braze flux residue.

Fig. 7: Mapping of potassium in the non brazed area of the joint.

The tube surface presented in Fig. 7 shows very uneven flux distribution. There are areas completely free of potassium (flux) and areas covered with a layer of flux. This would indicate that the tube surface was not completely wetted by flux upon application. Such “poisoning” of the surface can appear when it is contaminated with carbon. Though the overall level of carbon (as examined be SEM) is low, its distribution is uniform. The carbon most likely originated from binder traces. It seems that during debindering treatment the carbon got embedded into the aluminium oxide layer. Such a modified aluminium oxide layer is more difficult for the flux to remove.

5. Example of cost comparison calculation

The major reason behind the decision to introduce flux painting technology should be reduced overall cost of the manufacturing process. These should include:

Media

Maintenance

Environment (cost of waste utilization)

Raw material and consumables

Labor

Investment cost (depreciation)

In the following section we will present two examples of such calculations. It must be pointed out that the quoted numbers, though not far away from the values one can see in an aluminum brazing factory, may always differ from case to case. Thus the presented calculations should be considered only as a tool in which individual data needs to be fed, not as an indication which fluxing method is better.

Case A: Condensers

In the modified condenser production, the tube to fin joints are realized by tube precoating with NOCOLOK Si Flux3 – while the manifolds are prefluxed with a water based flux paint. This concept allows for complete elimination of the fluxer and the thermal degreaser from the brazing line. The dryer then acts as preheat and oven, where partial debindering takes place.

Fig. 8: Standard and modified process flow for condenser production.

Table 3 shows assumed input values like for example cost of electricity and the difference between the cost of standard process and modified one in the above listed categories. The whole calculation is done in an excel table where one can play with different input data which shows how sensitive the overall cost is to a change in a given parameter. In this case analyses showed that the most sensitive factor is the cost of the Sil Flux precoated tubes. Change of price in tube material by 10% can entirely reverse the final result.

Charge Air Coolers have one characteristic feature. Inside the tubes there are turbulators to make the flow of the hot compressed air more turbulent to increase the transfer of heat from the air to the tube walls. A condition to secure sufficient tube resistance to the inner pressure is to have all turbulators uniformly brazed to the inner tube surface. This requires fluxing of the tube’s internal surface. The most robust method is fluxing by immersion. This however produces higher post brazed flux residue levels and it is usually a bottle neck in the continuous production flow.

The other method is to apply a high pressure spray of flux slurry across the tubes. When combined with cross blow of high pressure air it secures proper inner fluxing even up to 900mm long tubes. This method does not slow down the production flow in a continuous line. For comparison the latter cross spray fluxing method is compared with prefluxing of turbulators. It should be remembered that wet fluxing is applied on tube to fin joints in both cases.

In this example there is no major change in the brazing line and as a matter of fact an additional entirely separate operation is added. This situation is somewhat improved when the flux painting process of the turbulators is incorporated into the tube making unit; however it eliminates only the negative effect of additional labor. The major cost is connected with consumption of flux paint. On average the cost of flux paint is about 20% higher than the cost of flux. Assuming liquid flux paint consumption of 4 grams per one turbulator and 50 grams of flux powder used on one charge air cooler fluxed by cross spray we will end up with such high difference in cost. In spite of this fact there are production lines which use the flux painting applied on turbulators.

6. Summary and conclusions

During recent years, the concept of prefluxing with binder flux/ paint flux has become quite popular as a method for fluxing. However when taking a decision about choosing this technology, the following aspects should be considered:

What level of adhesion is required?

Are we going to deal with binder removal from enclosed spaces?

Is the binder vapor going to affect my equipment?

and the most important one:

will there be real cost benefits?

In the case of cost calculations for NOCOLOK Sil Flux coated extruded condenser tubes the major factor influencing the overall cost is the cost of the coated tubes. Though the presented calculation is only an example and can slightly vary from case to case, it is the authors’ opinion that in certain cases the introduction of this technology can be justified by cost savings. In the case of flux painted turbulators for charge air coolers, the major factor influencing the overall calculation is the cost of flux paint. It seems that such process is not always justified from the cost point of view.

Over the last 15 years, selective pre-fluxing – also called paint fluxing or binder-based fluxing – has evolved as an alternative method for applying flux powder in aluminium brazing industry. There are many activities to define process parameters of fluxing with adhesives.

The first part of this paper outlines key features of pre-fluxing. The methodology for measurements of physical characteristics of binders and paint flux mixtures are described. General rules for behaviour of flux paints in brazing process are discussed together with some examples of flux paint features.

In the second part a case study is shown to illustrate common challenges when brazing with flux paint. The last part of this paper provides a cost comparison as guidance for choosing the right fluxing method for two different cases, one being extremely negative and the second as a positive case.

One of the most important characteristic of a binder is the kinetics of binder removal. This property is measured by a method called Differential Thermal Analysis [DTA]. The specimen (binder or flux paint) is placed in a small crucible and heated with a preset rate. The device measures the change of weight of the specimen and heat emitted or absorbed by the sample. The test can be done in air or at a chosen gas atmosphere.

The upper curve represents lost of weight upon heating, and the lower curve represents thermal effects appearing in the heated sample. The endothermic effect is associated with evaporation of the sample and the exothermic effect is usually connected with burning of the sample.

It should be observed that the above curves represent a sample of liquid flux paint. The removal of the the liquid phase (carrier evaporation) takes place during curing of the painted part. This process is always done before putting the parts into the brazing line. For flux paints made with water as a carrier it is simple evaporation.

Removal of the solid phase (cured binder) takes place at much higher temperature then evaporation of the carrier. It usually happens in the brazing line – in the dryer and partially in the brazer. Kinetics of the solid phase removal is shown in Fig. 2. In this case the analyzed sample is prepared by painting a metal surface, curing the paint and careful scratching off the solid paint, which is then analyzed in DTA device.

As can be seen the end of the binder removal is in the temperature range of 450oC. The above presented curves show the removal of binder at a constant heating rate of the sample (in this case 10oC/min). In the brazing line the prefluxed parts firstly go through a dryer where the temperature for dry parts is usually in between 200oC to 250oC. The parts for a continuous brazing line usually stay in the dryer no longer than 10 minutes.

To simulate this condition, a dry flux paint sample was analyzed by DTA with a hold for 10 minutes at 300oC. As can be seen from Fig. 3, holding at constant temperature for a prolonged time does not lead to full removal of the binder. In the given case only about 36% of the binder was removed.

Fig. 3: DTA curves with holding time 10min. at 300oC Test performed in air.

Different furnace design and different size of the brazed parts are responsible for different heating kinetics in the brazing lines. An influence of different heating rates on kinetics of binder removal is shown in Fig. 4.

Fig. 4: DTA curves with different heating rates Test performed in nitrogen.

The curves presented in Fig. 4 were obtained from analyzing a polyurethane binder heated in nitrogen atmosphere. It can be seen that only the middle temperature is moved to higher values with increased heating rate. The beginning and end of the debinding process do not depend on the heating rate.

Several examples of debinding temperatures for different type of binders are presented in table 2.

Binder Type

Tested in Air

Tested in Nitrogen

Middle temp. [°C]

End Temp. [°C]

Weight loss [%]

Middle temp. [°C]

End Temp. [°C]

Weight loss [%]

Polyurethane Binder A

355

530

99.7

370

460

99.6

Polyurethane Binder B

360

550

98.5

370

460

97.5

Acrylic Binder C (water soluble)

317, 382

450

87.2

220, 387

430

85.2

Acrylic Binder D (high adhesion)

267

420

Not measured*

385

Not measured*

27.5

Acrylic resin Binder E

275

400

86.9**

370

450

89.3**

* DTA performed only on ready mixtures
**Lower values due to some flux residue (sample obtained from ready mixture)

Table 2: Examples of debinding temperatures for different types of binders.

Over the last 15 years, selective pre-fluxing – also called paint fluxing or binder-based fluxing – has evolved as an alternative method for applying flux powder in aluminium brazing industry. There are many activities to define process parameters of fluxing with adhesives.

The first part of this paper outlines key features of pre-fluxing. The methodology for measurements of physical characteristics of binders and paint flux mixtures are described. General rules for behaviour of flux paints in brazing process are discussed together with some examples of flux paint features.

In the second part a case study is shown to illustrate common challenges when brazing with flux paint. The last part of this paper provides a cost comparison as guidance for choosing the right fluxing method for two different cases, one being extremely negative and the second as a positive case.

1. Introduction

In aluminium brazing industry fluxing is one of the most important steps in the production process. Flux water slurry application is considered as a standard and as a matter of fact very robust methodi. We believe that about 70% of overall fluxing activities are done by flux water slurry spray. This method has however certain disadvantages like troublesome slurry preparation and requirements for large and sometimes expensive machinery.

In order to lower production costs, new fluxing technologies have been introduced. One of them is called paint fluxing or binder-based fluxing which allows for elimination of wet fluxing process from the brazing line. Actually, the prefluxing process can be even done by an external company/subcontractor. It is nevertheless still a process of coating, which is done to particular component surfaces of the whole assembly – usually before the components are assembled.

In the industrial practise – particularly when one is in contact with many different users of the prefluxing technology – it is quite important to define the basic features and properties of discussed technology.

Flux Painting Booth

Flux Paint:
A mixture of brazing flux with binder, demineralised water or organic solvent, and thickener (the latter not always obligatory)

Binder:
Organic complex compounds, which are activated by curing – to provide adhesion of flux particles to the painted surface.

Thickener:
Organic substance, which is used to adjust viscosity and to facilitate re/mixing of the flux paint.

Curing:
Drying of the painted parts – usually with a blow of hot air (about 150°C). During that process liquid carrier (water or organic solvent) is evaporated and binder becomes activated to provide adhesion.

Adhesion:
Qualitative or quantitative measure by which strength of the flux particles bonding to the painted surface is determined. There are many different methods to describe adhesion of the flux paint. At Solvay we are using a simple quantitative method: A coated and cured coupon is placed in a holder positioned on a scale, a steel wedge is moved along the coupon with a gradual pressure increase. The weight at which the first scratches appear is a numerical value for adhesion.

Debinding:
Removal process of the binder from the painted surface done by treatment with high temperature, either in air or in ambient atmosphere.

Binder must be removed before reaching brazing temperature; otherwise the carbon residue will interfere with the brazing process – leaving both black stains on the part surfaces and very often leading to lack of brazing. Removal of the binder is done by applying high temperature to the assembled parts. In most cases the process of binder removal is realized in the brazing line both in the dryer and brazer.

2. Basic Rules for Flux Painting

The process of coating can be done by spraying, roller coating, brushing, or dipping. Uniformity of coating is very important, any agglomerates and lumps must be avoided. The flux paint can be prepared without thickener. Practically it is required in the mixture when a longer storage is expected (i.e. more then few days). There is an optimal temperature for curing resulting in maximum adhesion. Curing at ambient temperature is possible, but it will yield lower adhesion. Curing at too high temperature can lead to significant loss of adhesion.

Apart from the curing condition, adhesion also depends on the ratio of binder in the mixture (the more binder in the mixture the stronger adhesion), and the type of the binder (as a rule of thumb: the higher flash point of the binder the stronger adhesion). The most common method for prefluxing with binder mixture is using an atomized spray method on a machine which performs degreasing, painting and curing.